1. Field Of The Invention
The invention relates to a frequency stabilized passive Q-switch laser and, in particular, to a passive Q-switch laser that stabilizes its repetition rate by utilizing the optical external modulation.
2. Description Of The Prior Art
The passive Q-switch laser has many good merits over the active Q-switch laser, for example, it does not need high voltage power supply and high frequency electronic circuit, it is small in volume, and so on. However, the passive Q-switch lasers used to use dyes as the saturable absorber in early development, and therefore could not reach the high peak value in output power. In the recent years, the newly developed solid saturable absorber, the Cr:YAG crystal, can undertake a fairly large power ( greater than 500MW/cm2) and receives broad attention.
Nonetheless, conventional passive Q-switch lasers often suffer from the spontaneous emitted photons in the cavity and having unstable repetition frequencies that generate more than 20% of timing jitter. As shown in FIG. 1, the minimum average number of photons (xcfx86min) of stimulated emission in the cavity can have a great influence on the repetition rate of the lasers even it is extremely low. The following equation tells us the contribution to the generation of stimulated emission, xcfx86, in the cavity due to the spontaneous power, Psp:       Psp    ≈                            ϕ          xc3x97          h          ⁢                      xe2x80x83                    ⁢          υ                τ            ⁢                        4          ⁢          π                Ω            ⁢                        Δ          ⁢                      xe2x80x83                    ⁢                      W            spon                                    Δ          ⁢                      xe2x80x83                    ⁢                      W            laser                                ,
where hv is the photon energy, xcexa9 is the solid angle of the laser beam in the cavity, xcex94Wlaser and xcex94Wspon are the laser and spontaneous linewidths, respectively. The computer simulation results in FIG. 1 are taken by assuming a laser system consisting of a 5 mm-thick, 1 at % doped Nd:YAG crystal and a Cr4+:YAG crystal with 90% and 80% low power transmittances located in a half-symmetric laser cavity. This computer model has been compared and agreed with the experiments, as shown in FIG. 2.
Since the spontaneous radiation laser crystal, after stimulation, will generate the natural phenomena of timing jitter, which can not be filtered out, therefore there is no way to solve the problem of the unstable repetition rate of the passive Q-switch laser.
Thus, the above mentioned prior art still has many drawbacks. It is not a good design and needs modification. In observation of the many disadvantages in the prior art, the inventor sought to improve the technology and finally came up with this frequency stabilized passive Q-switch laser after many years of research and hardworking.
The invention provides a frequency stabilized passive Q-switch laser, where the volume of the semiconductor laser or the solid state laser stimulated by the semiconductor laser is small and consumes little electric power. Thus it is possible to achieve the effects of stabilizing and controlling the frequency almost without increasing the volume and power consumption of the passive Q-switch laser. Furthermore, the instant invention provides a frequency stabilized passive Q-switch laser, which utilizes the optical external modulation method to stabilize the repetition rate of the passive Q-switch, and thus to lower its timing jitter. At the same time, the repetition rate of the Q-switch laser can be controlled by the same technology to meet various application needs.
The frequency stabilized passive Q-switch laser with the above mentioned merits comprises an external amplitude-modulated semiconductor laser or a solid state laser stimulated by the semiconductor laser. The wavelength falls within the absorption wavelength range of the saturable absorber in the passive Q-switch laser so that the number of electrons in the ground and excited states of the saturable absorber can be modulated, and thus the repetition rate of the laser is stabilized by the external modulation.